This invention relates in general to shaft seals and in particular to a new and useful liquid sealed shaft with a partial seal gap being constructed as a threaded shaft seal.
This invention particularly concerns a shaft seal for sealing the lead-through between an inner space and an outer space of a shaft with feed bores for sealing liquid supplied at higher pressure than the pressure in the inner space, and partial seal gaps extending on both sides of these feed bores, with the partial seal gap on the side of the inner space being designed as a threaded shaft seal with recycling toward the sealing liquid feed.
Threaded shaft seals are known. They have the advantage over a seal with a smooth sealing gap that under the action of the increased pressure at the outlet of the feed bores compared to the inner space they produce recycling of sealing liquid flowing away from the feed bores over the inner sealing gap, by means of grooves incorporated in the shaft defining the seal gap or in the housing defining the seal gap at an angle to the axial direction, so that this sealing liquid leakage is limited. The sealing liquid entering the inner space in most cases is not usable because of contamination with the process gas in it.
However, this effect of recycling occurs only at higher speeds for which the grooves of the threaded shaft seal are designed. More severe leakage occurs when the shaft is stopped and at low speeds such as those that occur also in turbine startup in start/stop operation. An axial slip ring seal is known as a shutdown seal and a seal for low speeds, that has only slight leakage of sealing liquid. At higher speeds, however, high slip velocities are reached that lead to overheating and decomposition ofthe sealing liquid.
It is also known from German Patent No. 834 930 how to lift these slip rings away at higher speeds by centrifugal pressure produced by a rotating ring, with the seal then being produced by the rotating liquid ring.
This seal has the drawback that the rotating sealing liquid is severely heated because of the disk friction with the lack of liquid exchange.
The rotating liquid ring also forms a cylindrical interface with the adjacent process gas in the inner space which can lead to radial secondary flow with corresponding gas absorption in the sealing liquid, especially in the case of rotating liquid rings that are wide in the axial direction caused by the axial displacements of the shaft.